Autoimmune (idiopathic) thrombocytopenic purpura (ITP) is a disease caused by autoantibodies directed against platelets, but the demonstration of specific antibodies has been difficult for a variety of reasons. In general, when the antibodies can be demonstrated, there is an inverse correlation with the platelet count in individual patients. We have set up an assay for specific platelet glycoproteins, to aid in the diagnosis, treatment, and monitoring of patients with ITP. We are using the tests particularly for the followup of patients before and after treatment in 2 studies: with NHLBI in the treatment setting of T-cell-depleted auto-stem cell transplantation in patients with severe ITP, and in an ITP treatment protocol for less severely affected patients using Daclizumab. Sixteen patients have been studied in the transplant protocol and 4 have been studied in the daclizumab protocol. A publication regarding the transplant patients is listed below, and an abstract has been submitted to the American Society of Hematology (December, 2003 meeting) regarding the patients with daclizumab.